The present invention relates to a rare earth oxide superconducting material and a process for producing said superconducting material. More particularly, the present invention relates to a rare earth oxide superconducting material which comprises grains of an oxide superconducting material represented by REBa.sub.2 Cu.sub.3 O.sub.y (RE is Y, Gd, Dy, Ho, Er or Yb) and at least one element selected from Rh and Pt, uniformly dispersed in said grain and which gives a high critical current density even in a highly magnetic field, as well as to a process for producing said superconducting material by melting the raw materials.
Oxide superconducting materials have high critical temperatures and vigorous researches are under way in order to put them into practical use. In obtaining an oxide superconducting material as a bulk material, sintering has heretofore been used generally. The oxide superconducting materials obtained by sintering have such a microstructure as the grains: are fine and a number of grain boundaries exist inside; when observed under an optical microscope, they show, in some cases, the presence of cracks along the grain boundaries and impurities at the grain boundaries. In these oxide superconducting materials obtained by sintering, the superconducting grains are combined with each other by a weak-link, and the critical current density (Jc) is controlled by the weak-link, making it impossible to obtain a high Jc.
Meanwhile, in superconducting materials of single crystal structure, it is known that no grain boundary problems as mentioned above exist and that a high Jc is obtained even in a highly magnetic field. Hence, it was investigated to allow the superconducting material of microstructure obtained by sintering to approximate a single crystal structure and it was proposed to disperse microstructure particles of superconducting phase in a single crystal phase to fix the magnetic flux line coming into, that is, to form pinning centers. For example, a melting process represented by a MTG process (a Melt Textured Growth process) was proposed. In the MTG process, a rare earth oxide superconducting material, for example, is slowly cooled generally from the lncongruent melting point of 123 phase [REBa.sub.2 Cu.sub.3 O.sub.y (RE is a rare earth element including Y)] to give rise to a peritectic reaction between 211 phase (RE.sub.2 BaCuO.sub.5) and liquid phase to cause crystal growth; the 211 phase exists inside the crystals because of incomplete reaction after growth and acts as pinning centers; as a result, the rare earth oxide superconducting material obtained shows a high Jc even in a magnetic field. The oxide superconducting material obtained by the MTG process, however, has various disadvantages in that the particles of the 211 phase are large and their distribution is non-uniform and that cracks exist along the direction of crystal growth.
Also, a CG process (a Quench and Melt Growth process) was proposed in Japanese Patent Application Laid-Open No. 153803/1990, as an improved process for the MTG process. In tile QMG process, raw materials for rare earth oxide superconducting material are subjected to melting, quenching and solidification to obtain an intermediate comprising a Ba-Cu oxide phase and an Y.sub.2 O.sub.3 phase or the like of 50 .mu.m or less dispersed in said oxide phase, or Y.sub.2 O.sub.3 and a Ba-Cu oxide are mixed to obtain a plate-like material of 5 mm or less in thickness or a linear material; the intermediate or the plate-like or linear material is heated at the incongruent melting point of 123 phase to convert to a semi-molten state and then is cooled slowly from that temperature at a given cooling rate to give rise to a peritectic reaction between 211 phase and liquid phase to grow a 123 phase in which a 211 phase of 20 mm or less is finely and uniformly dispersed. According to the disclosure in the above document, the superconducting material obtained by the QMG process exhibits a very high pinning effect and gives an excellent Jc in a highly magnetic field. Further, a MPMG process (a melt powder and melt growth process) was proposed. In the MPMG process, a material obtained by melting, quenching and solidification according to the QMG process is ground for higher shapability and the obtained superconducting material is said to give the same high Jc as In the QMG process.
The QMG process and the MPMG process, as compared with the melt processing, can exhibit a high pinning effect and give an excellent Jc, but must conduct melting in two stages making the procedure complex.